1. Field of the Invention
The present invention relates to a resin composition comprising a polypropylene having a substantially syndiotactic structure, and more specifically, it relates to a polypropylene resin composition whose physical properties are scarcely deteriorated by radiation.
2. Description of the Prior Art
Polypropylene is inexpensive and has relatively suitably balanced physical properties, and therefore it is now used as materials for containers and packages of foods and medicines. In using polypropylene for these purposes, it is necessary to sterilize the containers and packages, and for this intention, irradiation is effected. The sterilization by the use of the radiation is easy but causes a decrease in the molecular weight and the deterioration of physical properties of the polymer.
A syndiotactic polypropylene has been known for a long period of time. Although such a polypropylene is prepared by polymerization at a low temperature in the presence of a conventional catalyst comprising a vanadium compound, an ether and an organic aluminum compound, the thus prepared syndiotactic polypropylene is poor in syndiotacticity and has characteristics like an elastomer. Thus, it is not considered that the polypropylene has syndiotactic characteristics.
A polypropylene having good tacticity, i.e., a syndiotactic pentad fraction of more than 0.7 in accordance with .sup.13 C-NMR has been discovered for the first time by J. A. Ewen et al. which can be obtained by polymerizing propylene in the presence of a polymerization catalyst comprising compounds of transition metals (Hf and Zr) having an asymmetric ligand and methyl aluminoxane (J. Amer. Chem. Soc., 110, 6255-6256, 1988).
In the J. A. Ewen et al. method described above, the activity of the transition metal is high, and the obtained syndiotactic polypropylene is excellent in syndiotacticity and physical properties inclusive of impact resistance. Therefore, this kind of syndiotactic polypropylene is on such a level as to be sufficiently utilizable in uses of a conventional isotactic polypropylene.
In recent years, the uses of polypropylene which require sterilization has increased, and the development of polypropylene is desired in which physical properties scarcely are deteriorated at the time of the sterilization by the radiation and which has an excellent balance of physical properties.